Resilient interconnection bridge

ABSTRACT

An elastomeric member is incorporated between a conductor and a conductive interconnection bridge. This elastomeric member serves as a spring providing compliance to an interconnection so that the flatness requirements of an opposing mating structure may be relaxed. A flat metal mandrel is provided with an elongated curved depression extending across a predetermined region where a resilient interconnection bridge is to be located. A nonconductive layer of Teflon is bonded to the surface of the mandrel. Grooves are ablated in a predetermined configuration down to the conductive surface of the mandrel using an excimer laser and a computer-controlled x-y table. Fineline electrical circuits are electrodeposited into the ablated grooves. The elongated curved depression is filled with a silicone material. The silicone material is permitted to cure to form a compliant elastomeric member having the shape of the elongated curved depression. An insulating backing is laminated onto the electrical circuits and the elastomeric member, and the completed resilient interconnection bridge is removed from the mandrel. Another method is used when the interconnection site is terminated on a circuit pad or on a high density linear connector configuration. In this method, elastomeric material is dispensed onto the circuit pad or along a line transverse to the high density linear connectors. After the elastomeric material has cured, electrical conductors are bonded thereacross to form raised compliant interconnection features.

This is a division of application Ser. No. 919,472, filed Jul. 24, 1992,now U.S. Pat. No. 5,261,158, which was a division of application Ser.No. 643,310, filed Jan. 22, 1991, now U.S. Pat. No. 5,180,311.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to the following related patentapplications assigned to the assignee of the present patent application:Ser. No. 580,758, filed Sep. 11, 1990 now U.S. Pat. No. 5,197,184, forThree Dimensional Electroformed Circuitry, by Crumly, Schreiber andFeigenbaum; and Ser. No. 580,749, filed Sep. 11, 1990 now U.S. Pat. No.5,233,157, for Laser Pattern Ablation of Fineline Circuitry Masters, byCrumly and Schreiber.

BACKGROUND

The present invention relates to connecting devices for high densitymultichip hybrid integrated circuit boards and, more particularly, to amethod of making fineline flexible circuits for interconnectingminiaturized fine pitch connectors.

In todays miniaturized high-density multichip hybrid integrated circuitboards plug and socket connecting devices are no longer used. Instead,fineline flexible circuits having raised surface features such as bumpsor dots are clamped in electrical contact with fine pitch etched circuittraces on the circuit boards. Interconnection systems of this type aredescribed in U.S. Pat. No. 4,125,310 to Patrick A. Reardon, II; U.S.Pat. No. 4,116,517 to Selvane, et al.; and U.S. Pat. No. 4,453,795 toMoulin. The connectors of these patents have a plurality of metallicraised features that protrude or project from the plane of the circuitconductors. These raised features may be pressed against either similarraised features or mating conductive connecting pads or etched circuittraces on a circuit board. The two circuits may be physically clampedtogether to press the features against one another thereby making firmand intimate electrical contact between the two circuits. Suchinterconnects typically employ 0.003 inch diameter features on flexiblecircuits that connect to 0.005 inch wide etched circuit traces on thecircuit boards. The interconnect density may be as high as 2800interconnects per square inch.

Multilayer printed circuit boards often have an uneven or irregularsurface in the region where the etched circuit traces are located. Tomake reliable connection, more stringent flatness requirements arerequired for the circuit board, or the complexity and weight of theclamping structure is increased to force a better connection between thewires in the flexible circuit and the etched traces on the printedcircuit board. However, higher pressure exerted by the clampingstructure tends to damage the substrate or to distort the flexiblecircuit so that the wires "swim" and registration of the fine pitchconnections is lost.

Accordingly, it is an objective of the present invention to providecompliance to a fineline interconnection so that the flatnessrequirements of the opposing mating structure may be relaxed. Anotherobjective of the present invention is the provision of a low cost, highdensity compliant interconnection system capable of compensating fordevice surface irregularities. A further objective of the presentinvention is to add compliance to any flexible circuit fabricated usingconventional methods. A still further objective of the invention is toprovide spring compliance to a fineline flexible circuit interconnectionthat permits reducing complexity and weight of a clamping structure usedin conjunction therewith. Yet another objective of the present inventionis the provision of an interconnection system that is more reliable,more compact and less costly. Still another objective of the inventionis to provide an interconnection system that distributes the forcerequired to make contact, thus reducing the possibility of damage to thesubstrate.

SUMMARY OF THE INVENTION

In accordance with these and other objectives and features of theinvention, an elastomeric member is incorporated between conductors andconductive interconnection bridges that form a high densityinterconnection system. This elastomeric member serves as a spring andprovides compliance to the interconnection so that the flatnessrequirements of the opposing mating structure may be relaxed.

To provide a compliant elastomeric member to a fineline, high densityflexible circuit fabricated by conventional methods, a special mandrelis provided. The mandrel is a flat, rectangular plate made of a metalsuch as steel. An elongated, curved depression is provided completelyacross the mandrel in the region where the interconnection bridges areto be formed. Preparatory to building up the flexible circuit, a layerof Teflon is bonded to the surface of the mandrel. The pattern for theflexible circuit is ablated into the surface of the Teflon in adirection orthogonal to the direction of the depression by a sharplyfocussed laser beam. The pattern for the flexible circuit is cutcompletely through the layer of Teflon to the surface of the steelmandrel. A pattern of conductors made of a material such as copper isthen electroformed, plated or electrodeposited on the mandrel. Then theelongated, curved depression is filled with a material such as silicone,for example RTV. After the RTV has cured, it forms a compliantelastomeric member behind the resilient interconnection bridges formedby the flexible circuit conductors deposited in the elongated curveddepression. An insulating backing is then bonded to the flexiblecircuit, and the completed fineline, high density flexible circuit isremoved from the mandrel.

To make a resilient interconnection feature where the interconnectionsite is located on a circuit pad, the following technique is used. Adollop of elastomeric material is dispensed on the pad. After theelastomeric material has cured, a bonder such as a wire bonder or ribbonbonder made by Hughes Aircraft Company is used to stitch over thecompliant elastomeric material. Due to the bonder's ability to attachgold wire, no additional processing such as plating is necessary. Thenewly-formed interconnection feature is ready to use.

In the case of a high density linear fine pitch connector, a slightlydifferent technique is used. Such fine pitch connectors are needed forsurface mount applications, and for terminations similar to card edgeconnectors. This technique involves the application of elastomericmaterial dispensed along a line to form a ridge crossing over all of thefine pitch circuit traces. After the elastomeric material has cured, abonder is used to stitch over the compliant elastomeric ridge at eachone of the circuit traces.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 is a fragmentary perspective view of a portion of a resilientinterconnection bridge constructed in accordance with the principles ofthe present invention;

FIG. 2 is a fragmentary perspective view of a portion of a mandrel usedto make the interconnection bridge of FIG. 1;

FIG. 3 is a side view of the mandrel of FIG. 2 showing a layer of anonconductive material bonded to the surface thereof;

FIG. 4 is a cross-sectional view of the mandrel of FIG. 3 taken alongthe lines 4--4 of FIG. 3;

FIG. 5 is a side view of the mandrel of FIGS. 2, 3 and 4 showing aresilient elastomeric member formed in a depression in the surfacethereof;

FIG. 6 is a side view of the mandrel of FIGS. 2, 3, 4 and 5 showing aflexible substrate or insulating backing built up thereon;

FIG. 7 is a side view of the resilient interconnection bridge of FIG. 1after it has been removed from the mandrel of FIGS. 2-6;

FIG. 8 is a plan view of a resilient interconnection feature formed onan existing circuit pad;

FIG. 9 is a side view of the resilient interconnection feature of FIG.8;

FIG. 10 is a plan view of a plurality of resilient interconnectionfeatures formed on an existing high density, linear, fine pitchconnector;

FIG. 11 is a side view of the resilient interconnection features formedon the linear connector of FIG. 10; and

FIG. 12 is a front view of the resilient interconnection features formedon the linear connector of FIG. 10.

DETAILED DESCRIPTION

Typically, multilayer printed circuit boards may have an irregularsurface along which is disposed a high density linear interconnect. Aclamping structure is adapted to force the linear interconnects intoelectrical contact with flexible circuits. Flexible circuits inaccordance with the invention are used to reduce the complexity andweight of the clamping structure, resulting in an interconnect systemthat is more reliable, more compact and less costly. It also reduces anddistributes the force required to make contact, in turn reducing thepossibility of damage to the interconnection system.

FIG. 1 is an enlarged, partly broken away perspective view of a portionof a resilient interconnection bridge 20 constructed in accordance withthe principles of the present invention. The interconnection bridge 20comprises a flexible substrate or insulating backing 21, a resilientelastomeric member 22 fastened to the backing 21, and a plurality offlexible circuit conductors 23 extending along the surface thereof.Although only two conductors 23 are shown in the fragmentary view ofFIG. 1, a typical resilient interconnection bridge 20 might have severalhundred fineline, high density close pitch flexible circuit conductors23. For example, there might be 269 conductors 0.005 inch wide, eachhaving a 0.005 inch space between conductors so that a 0.01 inch pitchis achieved. The elastomeric member 22 in conjunction with the flexiblecircuit conductors 23 takes the place of the raised features such asdots or bumps that are used in conventional interconnection systems. Theelastomeric member 22 serves as a spring providing compliance to theinterconnection so that the flatness requirements of the opposing matingstructure may be relaxed.

Flexible circuits similar to the resilient interconnection bridge 20shown in FIG. 1 are usually made by photolithographic and etchingprocesses using print and etch techniques. Conventional steps employedin such processes include covering a dielectric substrate with a layerof conductive material, and coating the conductive layer with aphotosensitive etch resist. A mask having a pattern of opaque andtransparent portions is applied over the resist and exposed to light.The resist is developed to leave a pattern corresponding to the mask,and then the conductive material is subjected to an etchant bath. Thedeveloped resist is then stripped leaving the desired pattern ofconductive circuit traces on the surface of the substrate.

It is a feature of the present invention that the foregoing conventionalprocesses are not used. Conventional photolithographic or etchingprocesses require an expensive class 10,000 clean room for environmentalcontrol. Furthermore, etching processes require an excessive number ofprocess steps and produce undercutting that results in conductors thatdo not have a precisely rectangular cross section.

FIG. 2 is a fragmentary perspective view of a mandrel 30 for making theresilient interconnection bridge 20 of the present invention by abuilding-up process rather than by an etching away process. The mandrel30 is a flat rectangular plate made of a metal such as steel. Anelongated curved depression 31 is provided that extends completelyacross the mandrel 30 in the region where the interconnection bridgesare to be formed. FIG. 3 shows a side view of the steel mandrel 30. Themandrel 30 is polished to a very smooth finish such as a 4 m finish, anda coating of a suitable nonconductive material 32 such as, for example,Teflon, is applied and bonded to the surface thereof. The thickness ofthe nonconductive material 32 is selected or predetermined to be equalto the height of the flexible circuit conductors 23 (see FIG. 1 ).Thereafter, a pattern of grooves is formed in the nonconductive material32 by ablation using a sharply focussed beam from a laser. Typically,the laser is mounted above a work table that is movable in twodimensions. The mandrel 30 is securely mounted on the table and thelaser beam is focussed on the nonconductive material 32. The table isdriven in the desired pattern by computer-controlled motors.

The laser employed is typically an excimer laser emitting pulses at awavelength of 248 nanometers having a duration of a few nanosecondseach. The laser is focussed to provide a substantially parallel beam ofexceedingly small dimensions at the surface of the nonconductivematerial 32. Typically, the laser can be focussed to a spot as small asone-half mil in diameter. Accordingly, the laser is capable of ablatingthe nonconductive material 32 to form grooves having a width of severalmils or less, and as small as one-half mil. The grooves ablated by thelaser extend entirely through the nonconductive material 32 to exposethe conductive surface of the steel mandrel 30. The depth of the ablatedgrooves is determined by the thickness of the nonconductive coating 32which, in turn, depends on the desired predetermined thickness of theflexible circuit conductors 23. The thickness of the nonconductivecoating 32 may be readily increased or decreased, if desired.

Having completed the computer-controlled x-y traverse of the ablatinglaser beam, the nonconductive material 32 has a desired pattern ofgrooves with walls substantially perpendicular to the exposed conductivesurface of the steel mandrel 30. The walls of the grooves are parallelto one another and to the axis of the laser beam which is perpendicularto the surface of the mandrel 30. Accordingly, the grooves in thenonconductive material 32 are of rectangular configuration, and ofprecisely predetermined dimensions with high resolution.

If deemed necessary or desirable, the exposed surface of the mandrel 30at the bottom of the grooves may now be treated to promote release ofthe circuit conductors 23 that are to be additively electroformed in thegrooves. As to the surface of the nonconductive material 32, nopreparation to promote release is necessary. If the nonconductivematerial 32 is a material such as Teflon, or the like, the surface ofthe nonconductive material 32 itself provides for ready release of aninsulating backing 21 that is to be laminated thereon. However, ifdeemed necessary or desirable, the surface of the nonconductive material32 may be treated to promote such release.

FIG. 4 is a fragmentary cross-sectional view taken along the lines 4--4of FIG. 3, showing one of the grooves in the nonconductive material 32filled with a conductive material to form one of the flexible circuitconductors 23. This electrically conductive material may be copper ornickel, or the like. The conductive material fills all of the groovesmade by laser ablation to form a predetermined conductive pattern offlexible circuit conductors 23 as indicated in FIG. 1. The conductivematerial is applied by an additive process such as electrolyticdeposition, electroless, electrophoretic or electrostatic deposition, orthe like. If deemed necessary or desirable, the exposed surface of theflexible circuit conductors 23 may be suitably treated to promotebonding to the elastomeric member 22 and the flexible substrate orinsulating backing 21 that are to be built up thereon.

Referring now to FIG. 5, which is a side view of the mandrel 30 as inFIG. 3, after the flexible circuit conductors 23 have been formed byfilling the grooves with a conductive material, the elongated curveddepression 31 is filled with a resilient elastomeric material such assilicone, RTV for example. This elastomeric material is allowed to cureto form the elastomeric member 22 shown in FIG. 1. The elastomericmember 22 provides a springy elasticity behind the interconnectionbridge 20 formed by the flexible circuit conductors 23 deposited orplated in all grooves traversing the elongated curved depression 31.Referring to FIG. 6, the flexible substrate or insulating backing 21 isbonded to the flexible circuit conductors 23 and to the resilientelastomeric member 22. The insulating backing 21 may be formed of anyone of a number of suitable materials. These include polyimide,polyimide layered acrylic adhesive, polyethylene, polyester and vinyl.One example of an insulating backing 21 that has been found satisfactorycomprises a one mil layer of a polyimide covered with a one or two millayer of acrylic adhesive. The insulating backing 21 is bonded to theelastomeric member 22 and the deposited or plated circuit conductors 23under a pressure of about 300 psi. and at a temperature of about 370degrees Fahrenheit. At this pressure and temperature, the insulatingbacking 21 flows and enters the microstructure in the deposited orplated circuit conductors 23 which insures its adhesion thereto.

FIG. 7 shows the completed resilient interconnection bridge 20. A knifeedge or the like is inserted between the insulating backing 21 and thenonconductive material 32 to lift the insulating backing 21 togetherwith the resilient elastomeric member 22 and the flexible circuitconductors 23. The resulting resilient interconnection bridge 20comprises the flexible substrate or insulating backing 21 and resilientelastomeric member 22 on the surface of which is formed and bonded apattern of fineline, high density, close pitch flexible electricalconductors 23. The elastomeric member 22 protrudes to form, along withthe conductors 23 that traverse it, a raised interconnection feature.

It will be understood that the mandrel 30 is reusable and can be used tomake as many of the resilient interconnection bridges 20 as desired. Thereusable master or mandrel 30 enables manufacture of electricalcircuitry by fully additive electrodeposition processes, allows moreprecise control of placement dimensions of conductive traces orcircuits, and completely eliminates conventional photoetching andstripping operations. By eliminating photo-developing, etching andstripping operations, the hereindescribed additive processes requiremany fewer process steps. As a result, costs of manufacture aredramatically reduced and speed of production increased. The hereinabovedescribed method is efficient and provides a circuit free from inherentdistortions found in circuits produced by etching. Conventionalsubtractive processes have a thermal coefficient of expansion mismatchbetween dielectric and copper that is etched away. Furthermore,environmental control is not as critical as in photolithographic oretching processes. The additive process described hereinabove producescircuit features in a relatively inexpensive, easy to maintain platingshop environment. Photolithographic processes, on the other hand,require an expensive class 10,000 clean room.

The process described herein controls the configuration of theelectrical circuit by computer-controlled x-y positioning of the mandrelbeneath a laser beam. Accordingly, the configuration of the electricalcircuitry can be readily changed to provide a different pattern and adifferent circuit by merely changing the computer software. In thephotolithographic method, on the other hand, it is necessary to designand manufacture new artwork, and new masks.

Referring now to FIGS. 8 and 9 of the drawings, there is shown a planview and side view, respectively, of one embodiment of a resilientinterconnection feature 110 formed on an existing circuit pad 113. FIGS.8 and 9 show a substrate 111 made of a suitable insulating material suchas polyimide. On the substrate 111 is a conductive circuit trace 112which may be made of copper, for example, and which terminates in acircuit pad 113. In making the resilient interconnection feature 110,first an elastomer is dispensed on the pad 113 to form an elastomericdollop. The elastomer may be made of silicone, and may be a product suchas RTV. The elastomer is permitted to cure to form an elastomeric member114 which serves as a spring and provides compliance to the resilientinterconnection feature 110. After the elastomeric member 114 has cured,gold wires 115 are stitched over the compliant elastomeric member 114 toform a bridge in two planes. A wire or ribbon bonder of the typemanufactured by Hughes Aircraft Co. may be employed to bond the goldwires 115. Due to the bonder's ability to attach the gold wire 115, nofurther processing such as plating is required, and the resilientinterconnection feature 110 is ready to use. The invention may beemployed with any flexible circuit fabricated using conventionalmethods.

Referring now to FIGS. 10, 11 and 12 of the drawings, there is shown aplurality of interconnection features formed on an existing highdensity, linear, fine pitch connector 130. FIG. 10 shows a plan view,FIG. 11 shows a side view, and FIG. 12 shows a front view. Thisembodiment of the linear fine pitch connector 130 is exemplary of a highdensity linear interconnect arrangement. Such arrangements are typicallyfound in surface mount technology or, if folded over, it isrepresentative of arrangements typically found in a card edge connector.FIGS. 10, 11 and 12 show a substrate 131 made of a suitable insulatingmaterial such as polyimide, for example. On top of the substrate 131there are disposed two circuit traces 132 which are made of a conductivematerial such as copper, for example. A first insulating layer 133covers a portion of the traces 132, while a second insulating layer 134covers a different portion of the traces 132. For purposes of clarity,neither one of these insulating layers 133, 134 is shown in FIG. 12. Thefirst and second insulating layers 133, 134 are made of a suitableinsulating material such as polyimide, for example.

The first and second insulating layers 133, 134 cover the circuit traces132 except for a space between the ends of the insulating layers 133,134 where the traces 132 are exposed. An elongated elastomeric member135 is disposed transversely across the exposed portion of the circuittraces 132. The elongated elastomeric member 135 is formed by dispensingan elastomer transversely across the exposed portion of the circuittraces 132. The elastomer may be made of silicone, for example, and maybe a product such as RTV, for example. The elastomer material ispermitted to cure to form the elastomeric member 135 which serves as aspring and provides compliance to the interconnection feature. After theelastomeric member 135 has cured, gold wires 136 are stitched over thecompliant elastomeric member 135. A wire or ribbon bonder of the typemanufactured by Hughes Aircraft Co. may be employed to bond the goldwires 136. Due to the bonders' ability to attach the gold wires 136, nofurther processing such as plating is required, and the interconnectionfeature is ready to use.

A novel feature of this invention is the presence of an elastomericmember incorporated between the conductor and conductive interconnectionbridge. This elastomer serves as a spring providing compliance to theinterconnection so that the flatness requirements of the opposing matingstructure may be relaxed. This is a desirable feature when attempting tomate to multilayer PC boards which often have an irregular surface. Thedescribed invention provides a high density interconnection system withan integral spring.

The present invention reduces the complexity and weight of the clampingstructure, resulting in an interconnect system that is more reliable,more compact and less costly. It also reduces and distributes the forcerequired to make contact, in turn reducing the possibility of damage tothe substrate.

In practice, the invention has been utilized for bonding 0.003 inchribbon to a 0.005 inch wide circuit trace. The length of the bridge isapproximately 0.025 inches. Given these dimensions, a .01"×.035" pitchis achievable, for an interconnect density of 2800 interconnects persquare inch.

Thus there has been described a new and improved resilientinterconnection bridge. It is to be understood that the above-describedembodiments are merely illustrative of some of the many specificembodiments which represent applications of the principles of thepresent invention. Clearly, numerous and other arrangements can bereadily devised by those skilled in the art without departing from thescope of the invention.

What is claimed is:
 1. Apparatus for use in manufacture of a resilientinterconnection bridge comprising:a mandrel including a mandrelsubstrate having a surface formed of a material that acceptselectroforming of electrically-conductive circuit traces; a protectivecoating of a material resistant to electroforming formed on said mandrelsubstrate, a pattern of grooves formed in said coating extending throughsaid coating to said mandrel, said grooves having walls that aresubstantially perpendicular to said mandrel substrate, whereby a patternof electrical conductors is electroformed upon said mandrel substrate insaid grooves, and whereby a dielectric substrate is laminated upon saidcoating and conductors to enable removal of the dielectric substrate andconductors from the mandrel substrate and its coating; and an elongatedcurved depression disposed across said mandrel in a predetermined regionwhere a resilient interconnection bridge is to be formed in which acompliant elastomeric member is formed.
 2. The apparatus of claim 11 inwhich said mandrel is reusable, and in which said mandrel comprises amandrel body having a mandrel surface defined by a first pattern ofelectrically conductive surface areas separated by a second pattern ofelectrically nonconductive surface areas, and in which said elongatedcurved depression is a feature formed in said mandrel at a predeterminedregion of said electrically conductive surface areas, said elongatedcurved depression defining a mold, whereby an electrical circuit iselectrodeposited upon said mandrel body over said predetermined region,and whereby a compliant elastomeric element is molded thereto to form araised compliant interconnection feature.